1. Field of the Invention
One embodiment of the present invention relates to a carbazole compound. In addition, one embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each of which uses the carbazole compound.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence (EL) have been actively conducted. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance.
Such a light-emitting element is of self-luminous type, and thus has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not needed, and so on. Therefore, such a light-emitting element is regarded as being suitable as a flat panel display element. Besides, such a light-emitting element has advantages in that it can be fabricated to be thin and lightweight, and has very fast response speed.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission easily; thus, large-area elements using planar light emission can be formed. This is a feature that is difficult to obtain for point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Therefore, the light-emitting element is very effective for use as a surface light source applicable to a lighting device and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as a light-emitting substance, by application of voltage to a light-emitting element, electrons and holes are injected into a layer containing the light-emitting organic compound from a pair of electrodes, whereby current flows. Then, with these carriers (i.e., electrons and holes), the light-emitting organic compound is brought into an excited state. The light-emitting organic compound returns to the ground state from the excited state, thereby emitting light.
Because of such a mechanism, the light-emitting element is called a current-excitation light-emitting element. Note that the excited state of an organic compound can be a singlet excited state (S*) and a triplet excited state (T*), and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence. The statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3. Accordingly, a light-emitting element including a phosphorescent compound is thought to be able to have emission efficiency three to four times that of a light-emitting element including a fluorescent compound. In recent years, a highly efficient light-emitting element including a phosphorescent compound has been actively developed.
In a light-emitting element including such a phosphorescent compound, a light-emitting layer is often formed such that the phosphorescent compound is dispersed in a matrix of another compound in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound. Here, the compound serving as the matrix is called host material, and the phosphorescent compound or the like dispersed in the matrix is called guest material.
Note that in the case where a phosphorescent compound is used as a guest material, a host material is needed to have larger triplet excitation energy (an energy difference between a ground state and a triplet excited state) than the phosphorescent compound. In addition, the use of an optimum host material for a guest material is needed in developing element characteristics of the light-emitting element. Therefore, in development of a highly efficient light-emitting element including a phosphorescent compound, development of a novel substance that can be used as a host material for the phosphorescent compound is also extremely regarded as important (e.g., Patent Document 1).